The present invention comprises a small capacity, single-processor version of the digital PBX--key system shown by U.S. patent application No. 135,464 filed on Mar. 31, 1980 by F. Ahmed and a second application, now U.S. Pat. No. 4,339,633 issued July 13, 1982 to F. Ahmed.
The system shown by the cited applications is a modular system comprised of plural groups of stations and trunks, with each group having a group processor, the group processors all having access to a central processor. Information between group processors and the central processor is routed by way of a memory in the group processors which stores messages until the receiving processor signals that it is ready to receive the stored information. By this approach, the need for interrupts is minimized.
In the system shown by the cited applications, all information within the system is routed over a systems bus with a further bus provided to transfer information between the processor and its memory, timers and decoders, this latter memory being called a nucleus bus.
Each station has four conductors extending between the station and an interface circuit performing the known supervisory functions of a line circuit. Of the four, two conductors are speech or data conductors and two are control conductors. The speech conductors receive speech and dialing information in analog form from the station and feed the analog data to inputs of a codec associated with the interface for that station. In the codec, the analog speech is translated into a PCM code and sent out on the group bus during a time slot. Naturally, the codec translates PCM code data to analog speech signals for transmission to its station.
The control conductors receive and transmit data in asynchronous digital form to stations of the type shown by U.S. Pat. No. 4,315,110 issued Feb. 9, 1982 to T. M. Davis. The control data is received in the station interface circuit to provide status information concerning the station condition.